Web Animations CSS: Support animation of stroke-dasharray property

The Web Animations implementation matches Firefox in all details except for
when animating to or from a value of 'none'. In this case, Firefox does not
animate, and the computed style is always 'none'. The Web Animations
implementation considers 'none' equivalent to a value of '0 0', as both result
in a solid line being drawn, and animates using an effective value of '0 0'.

The legacy implementation has a number of bugs ...
- The behavior when animating to or from a value of 'none' is odd.
- Values should be clipped to be non-negative. See http://crbug.com/305070.
- When lists consist of differing numbers of elements, we should repeat the
  lists to form a list of length equal to the lowest common multiple of the
  lengths of the individual lists. Currently we use a list of length equal to
  the multiple of the lengths of the individual lists.

BUG=257591
R=dstockwell

Review URL: https://codereview.chromium.org/26292004

git-svn-id: svn://svn.chromium.org/blink/trunk@159552 bbb929c8-8fbe-4397-9dbb-9b2b20218538